Indonesian climate investigations using stable isotopic data from modern, Holocene and Pliocene corals
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Earth’s climate system has experienced significant changes throughout its history. Tropical, glacial and interglacial conditions have persisted across the globe during the Tertiary, and the present interglacial period with strong seasonality and episodes of warming and cooling has existed for the last several thousand years. However, rapid warming during the last century has highlighted the importance of understanding past climate behaviour, in order to predict the likely effects of such warming on our future climate. This has led to the use of proxies, such as sediment cores, tree rings, ice and carbonates, which provide high resolution palaeoclimate archives. Studies using a variety of proxies from around the globe have developed a network of site specific climate information, offering insights of climate fluctuations on a millennial scale, and identifying changing oceanic conditions as a major influence on global climate change. Coral skeletons are particularly sensitive recorders of ambient seawater conditions as they record fluctuations of oxygen and carbon in their aragonitic skeletons, which are caused by perturbations of sea surface temperature (SST) and sea surface salinity (SSS). Therefore, tropical oceans themselves provide a crucial record of climate change. Many of the coupled atmospheric-oceanographic processes that drive the global climate system occur in the Indo-Pacific, making it a particularly important region for data collection. Porites corals from the Indo-Pacific have been studied in detail and have revealed high resolution SST and SSS records, yet comprehensive investigations using other coral species are minimal. This study expands upon the current understanding of coral proxies by investigating three coral species of different ages, and provides new insights into the palaeoclimate history of the Indo-Pacific. Goniastrea retiformis, Platygyra pini and Platygyra lamellina corals have been retrieved from Timor Leste in the Indo-Pacific, and analysed using stable and radiogenic isotopic techniques. Sclerochronology was used to determine that the 4.5 year old, modern G. retiformis coral was living from 2006-mid 2010. Uranium-thorium (U/Th) analysis provided a mid Holocene age of 4.5 ± 0.092 (2σ) ka for the fossil P. pini. Uranium-lead (U/Pb) techniques were used to obtain a 2.7 ± 0.34 (2σ) Ma age for the fossil P. lamellina. A rigorous diagenetic screening process of X-ray diffraction (XRD), petrographic analysis of thin sections, and scanning electron microscopy (SEM) has been carried out to ascertain the extent of preservation in each coral. X-ray images allow annual density bands to be located and unique micromilling paths were developed for the three samples. This enabled us to obtain the first δ¹⁸O and δ¹³C stable isotope results for a G. retiformis coral, and the first fossil coral results for the Platygyra genus. Cross spectral analysis has been used to verify the periodicity of seasonal fluctuations visible in the data, and confirmed that these coral species are suitable for use as climate proxies. G. retiformis is an abundant reef coral distributed throughout the tropical Pacific, and has a robust skeletal configuration making it suitable for use as a climate proxy. The 4.5 year long record has revealed that δ¹⁸O and δ¹³C values range from -4.62‰ ± 0.03‰ (2σ) to -6.11‰ ± 0.02‰ (2σ), and 1.57‰ to -2.53‰, with means of -5.35‰ and -1.02‰, respectively. These values are in agreement with the average δ¹⁸O range of -5.1 to -5.6‰ in modern corals throughout the Indo-Pacific, predominantly of the Porites genus. Statistical analysis of the isotopic data has revealed a quasi-biennial signal in G. retiformis, typical of the temporal interaction between El Niño Southern Oscillation (ENSO) and the South Asian monsoon. Results have also been compared to the Southern Oscillation Index (SOI) in order to assess potential relationships between coral isotopes and ENSO. This has shown an accurate record of El Niño and La Niña events for the first half of the ~ 4.5 year record, from 2006 to mid 2008. Climate processes such as local rainfall and the Indian Ocean Dipole (IOD) are also reflected in the record, to varying extents. The use of Platygyra corals as climate proxies is limited to three investigations using modern samples, despite the fact that this genus has a much broader latitudinal range than the extensively studied Porites genus. This thesis provides the first examination of fossil samples, and provides analysis of both pristine and altered specimens. The 4.5 ka P. pini coral has allowed investigation of the palaeoclimate record in a sample that has been exposed to diagenesis. XRD, petrographic and SEM analyses have revealed widespread secondary aragonite growth, dissolution and secondary calcite within the coral, which would have occurred due to marine and freshwater diagenesis. However, primary growth textures have been retained in some areas of the coral. δ¹⁸O and δ¹³C values range from -3.47‰ ± 0.03‰ (2σ) to -5.45‰ ± 0.02‰ (2σ), and 3.43‰ to -0.49‰, with means of -4.82‰ and 1.12‰, respectively. These values are significantly more positive that the mean δ¹⁸O of four Platygyra corals from the Pacific region, which range from -4.82‰ to -5.10‰. There are two reasonable explanations for this. Secondary aragonite, which is detected throughout the milled section of coral, causes positive shifts in coral δ¹⁸O, an effect which is likely to have altered the geochemical record of this particular coral. But the fact that SST in the southern Indo Pacific Warm Pool (IPWP) during the mid Holocene were 1.2° C cooler than present must also be acknowledged, as cooler temperatures also impart a shift toward higher δ¹⁸O, due to equilibrium fractionation processes. SST reconstruction using the temperature dependence equation from a modern Platygyra coral reveals an SST range of 18.5-27.6° C, and mean of 24.7° C. The present day mean annual SST of 28° C suggests that mid Holocene temperatures less than 26.8° C are unrealistically cool, highlighting the fact that isotopic fractionation during diagenesis has affected coral chemistry. However, the presence of interannual periodicity indicates that ENSO was operating, and the magnitude of isotopic fluctuation through the 10 year record is similar to that found in modern and other Holocene corals. We suggest that although absolute isotopic and SST values are unreliable, prohibiting the extraction of high resolution climate records, insights into the behaviour of broad scale, seasonal and interannual climate processes may still be obtained. A strong annual periodicity has been detected when analysing the stable isotopic values recorded in the 2.7 Ma P. lamellina coral. This indicates that seasonal SST fluctuations were the dominant influence on this coral. Ranges of -4.67‰ ± 0.03‰ (2σ) to -5.48‰ ± 0.02‰ (2σ) and 0.88‰ to -1.12‰ for δ¹⁸O and δ¹³C, respectively, are similar to modern Platygyra coral results, suggesting that this coral has been preserved in pristine condition. Palaeo-SSTs have been reconstructed using a modern Platygyra temperature dependence equation, providing a range of 24-27.7° C and a mean of 25.9° C. Foraminiferal data from sediment cores in the greater Indo-Pacific suggest that mean annual SSTs at this time were ~ 2-3° C cooler than present. The coral record I present supports this statement, providing new insights into our understanding of tropical palaeoclimates. This coral has been entrained within a turbidite deposit on the sea floor that has subsequently been uplifted during the emergence of Timor, with U/Pb dating allowing further constraints of the stratigraphic age of the deposit. Tectonic narrowing is postulated to have caused major changes to the Indonesian Throughflow (ITF) from 4-2 Ma, and been a driver of major global climate change beginning in the late Pliocene. This means that fossil Platygyra corals Timor Leste could provide unique time slices of information about this important time in global climate history. This study confirms that G. retiformis, P. pini and P. lamellina corals are excellent candidates for further, detailed investigations. They provide the opportunity to develop new coral proxies which are both abundant throughout the tropics and distributed over a wide latitudinal range. Their prevalence in both modern and fossil reefs means that once modern samples of each species have been calibrated against modern SST and SSS, these corals will provide reliable, quantitative palaeoclimate proxies, with potential for data capture throughout the Indo-Pacific and mid latitudes. Geochemical coral archives are a crucial tool in the study of climate processes, and we believe that these species are ideally suited to enhancing and refining our current understanding of earth’s climate system.